Low resistance ohmic contacts, particularly to many semiconductors with a band gap greater than 1 eV, are problematic for a number of well-known reasons:
First, metal/semiconductor contacts typically exhibit a large Schottky barrier height because the work function of the metal often places the metal Fermi level somewhere near the middle of the band gap. Also, Fermi level pinning by surface states also tends to pin the surface Fermi level near mid-gap. Both of these effects tend to make the Schottky barrier heights larger than 0.5 eV. Such large Schottky barrier heights prevent the formation of simple ohmic contacts where the Fermi level in the metal makes direct (i.e. barrier free) contact to the semiconductor conduction or valence band.
Second, tunneling ohmic contacts are problematic because it is difficult to produce extremely heavy doping in semiconductors with a band gap larger than 1.0 eV, as described in co-pending U.S. patent application Ser. No. 10/277,352, filed Oct. 22, 2002 (which is incorporated herein by reference). Heavy doping is required for a tunneling ohmic contact because heavy doping supports narrow depletion regions, which increase the probability for tunneling through the depletion region into the semiconductor. Heavy doping is often more difficult for one conductivity type (i.e. p-GaN contacts are much harder than n-GaN contacts because it is hard to achieve heavy p-type doping of GaN, while achieving heavy n-type doping is significantly easier).
Consequently, a general solution that would enable lower resistance ohmic contacts to be achieved for most wide band gap semiconductors would be highly desirable.
Lower resistance contacts are important because device performance can be improved by lowering power losses at contacts, which also tends to improve contact reliability. For example, light emitting diodes (LEDs) produced using GaN usually exhibit an excess voltage drop due to the resistance of the contacts. A significant component of such voltage drops can be attributed to the high resistance of typical p-GaN contacts. This excess voltage drop increases the power dissipation of the LED and ultimately limits the output power. Furthermore, low resistance p-GaN contacts enable a higher current density to be used before significant contact degradation occurs, thus resulting in improved performance and the capability to operate at higher current densities. Similarly, laser diodes produced in such wide band gap materials exhibit similar power and performance losses due to high resistance p-GaN contacts, and would therefore benefit from improved contacts.